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Operations and Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Inventories [Abstract]          
Fuel $ 25.3   $ 25.3   $ 28.0
Materials and Supplies 45.7   45.7   41.8
Total Inventories 71.0   71.0   69.8
Prepayments and Other Current Assets [Abstract]          
Deferred Fuel Adjustment Clause 18.3   18.3   22.5
Other 7.9   7.9   11.1
Total Prepayments and Other Current Assets 26.2   26.2   33.6
Other Current Liabilites [Abstract]          
Customer Deposits 29.1   29.1   28.8
Other 31.7   31.7   33.8
Total Other Current Liabilities 60.8   60.8   62.6
Other Non-Current Liabilities [Abstract]          
Asset Retirement Obligation 81.6   81.6   77.9
Other 47.7   47.7   45.4
Total Other Non-Current Liabilities 129.3   129.3   123.3
Supplemental Statement of Cash Flows Information [Abstract]          
Cash Paid During the Period for Interest - Net of Amounts Capitalized     22.8 21.7  
Cash Paid During the Period for Income Taxes     0.6 0.2  
Noncash Investing and Financing Activities [Abstract]          
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment     28.2 14.8  
Capitalized Asset Retirement Costs     1.9 2.4  
AFUDC - Equity $ 1.0 $ 1.2 $ 2.1 $ 1.9